The present invention relates to a method of processing an image signal produced as an output signal from a photoelectric transducer device used in a facsimile receiver/transmitter, a platemaking image reading apparatus, or the like.
Image scanning reproducing systems are widely used in the printing and platemaking industries for electrically processing image information of originals or subjects to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reproducing systems are basically constructed of an image reading apparatus and an image recording apparatus. In the image reading apparatus, image information of an original or subject which is fed in an auxiliary scanning direction by an image reader is scanned by image sensors in a main scanning direction substantially norma to the auxiliary scanning direction, and the scanned image information is converted into an electric signal.
Then, the photoelectrically converted image information is processed for gradation correction, edge sharpness, etc. according to platemaking conditions. Thereafter, the processed image signal is converted into a light signal such as a laser beam signal, which is applied to and recorded on an image recording medium of a photosensitive material such as a photographic film. The image recording medium with the image recorded thereon is developed by an image developing device and will be used as a film plate for printing.
The image sensors of the image reading apparatus generally comprise a plurality of series-connected CCD image sensors each of which is composed of an array of thousands of photoelectric transducer elements known as charge-coupled devices (CCDs).
More specifically, the image sensors are connected optically in series in order to read, with a high resolution, an original which is wide in the main scanning direction. Image information signals, which are obtained by photoelectric conversion using the image sensors, are electrically combined (added) into an electric image signal which corresponds to the original image information.
Even when no light falls on the photoelectric transducer elements, they develop a voltage at a dark level based on a dark current which flows in the semiconductor devices. The dark-level voltage varies from image sensor to image sensor because the dark current flowing in the photoelectric transducer elements varies from image sensor to image sensor.
When the output signals from adjacent image sensors are added, there is developed a level difference between the combined output signals, and such a level difference results in an unwanted level shift in the image signal. As a consequence, an image which is reproduced from the image signal suffers brightness irregularities, which manifest themselves particularly if the original image is high in density.